The present invention relates to power supply circuitry and particularly to power supply circuitry including a line filter for reducing the coupling of power supply generated interference into the power line and other circuits connected thereto.
Electronic power supply circuits may include means, or supply power to means, characterized by the generation of electromagnetic interference. For example, a power supply may drive an inverter for changing a DC voltage to an AC voltage and ultimately to another DC voltage value. Such inverter includes a square-wave generator operating at a rate which causes electromagnetic interference (EMI) at frequencies appreciably higher than the customary 50 or 60 cycle power line frequency. This interference is apt to be coupled directly through the power supply back into the line and to other apparatus connected to the same line.
Some areas of the world have standards which limit the amount of electromagnetic interference that a given device may couple into the power lines, particularly in high frequency ranges. Therefore, line filters are commonly employed in the power supply circuitry for rejecting the coupling of high frequency signals. A common form of such a filter includes series connected inductances in the power supply branches, and shunt connected capacitors disposed either between lines or between a line and ground. Consideration of "ground" or a "point of common reference potential" as a part of the circuit is particularly important in rejecting common mode EMI, i.e. interference that is generated by the offending circuitry with reference to ground. Thus, EMI is typically generated through voltage division of an inverter square-wave across the line impedance and the stray capacitance between ground and the active inverter components. The whole power supply circuit essentially forms one conductor for coupling EMI back into the line with respect to common ground as the other conductor.
The above indicated shunt line filter capacitances between the power supply and ground are helpful in alleviating this problem. Unfortunately, the size of these capacitances is limited by safety requirements and limitations on leakage current at the power line frequency. Thus, excessive leakage current from the power supply to chassis or cabinet ground could impose a hazard to personnel in the event the cabinet itself is not properly connected to earth ground. Since the size of grounding capacitors is limited, reliance on series inductors is emphasized and a line filter composed of a number of series inductances may be required in order to correct the EMI problem. These inductors are not only expensive but also tend to be large and space consuming.